1. Field of the Invention
This invention relates to cooled mirrors for use in laser systems and, in particular, to cooled mirrors in which a liquid is circulated through the mirror to remove excessive heat.
2. Summary of the Prior Art
High energy laser systems frequently require the use of mirrors to control the pointing direction of the laser beam. Usually the mirrors used in high energy laser systems are required to be cooled to prevent distortions from being introduced into the laser beam due to thermally induced distortion of the laser mirror's reflecting surface, and to prevent physical damage of the laser mirror from heat absorbed from the laser beam. Cooling traditionally is accomplished by circulating a coolant through a manifold in the supporting structure of the mirror and/or through the mirror's reflecting surface. See for example, U.S. Pat. Nos. 3,909,118; 4,443,059 and 4,770,521. As the power container in the laser beam increases, the amount of heat which must be removed from the mirror also increases, thereby requiring the movement of a greater mass of coolant at a higher velocity to achieve effective cooling of the mirror's reflecting surface. The high flow rates associated with such cooling introduces an undesirable effect in the mirror's reflecting surface known as coolant-flow induced jitter. This jitter component reduces the quality of the reflected laser beam at the image or focal plane and makes high resolution pointing and tracking of a high energy beam difficult to achieve. This problem is especially apparent in laser systems utilizing large diameter beams in which the surface which must be cooled requires complex manifolding and/or the movement of a conserval volume of coolant to effectively provide uniform thermal distribution across the entire face of the mirror. As the surface area of the mirror increases, differential thermal expansion may be experienced which produces bowing of the mirror structure due to localized heating across the surface. Traditionally, cooled mirrors have been fabricated from metals such as molybdenum or copper to take advantage of the high thermal conductivities of these metals. Unfortunately, mirrors manufactured from metal such as the foregoing are relatively heavy and bulky and are thus not suitable for use in space-based applications.
Lighter weight mirrors may be fabricated from materials having low coefficients of thermal expansion, such as zerodur type glass, silicon or silicon carbide. While use of materials such as the foregoing reduces the weight of the mirror structure, the detrimental effect of localized heating of a high energy laser beam, namely, beam degradation due to thermal distortion effects, are not eliminated. Thermal degradation still must be controlled by the use of coolant circulated through the mirror's structure, without the coolant inducing jitter from movement of the coolant through the structure. In addition, the coolant system should still be capable of selectively handling localized increases in the thermal load across the face of the mirror due to non-uniformity in the power density of the laser beam reflected from the mirror's surface. Ideally, the coolant system used for handling heatloading of the mirror should be selectively adaptable to provide only as much coolant as is required to handle the absorbed heat in localized portions of the mirror, without requiring excessive flow volume or pressure to minimize the effects of flow-induced jitter in the laser beam deflected from the mirror's reflecting surface. Using a conventional cooling approach, i.e. where coolant is uniformly passed through the entire mirror structure to handle the highest heatloading at any given area of the mirror, coolant flow rate must be achieved to control thermal distortions at the hottest spot on the mirror's surface. Systems which solve the heating problem utilizing the foregoing approach do not adequately address the problem of reducing flow induced jitter in the reflecting surface, nor are they entirely suitable for use in light weight structures where the material used to produce the mirror and its associated support structure may not have a high coefficient of thermal conductivity due to the need to produce the mirror and support structure from as low weight material as possible, particularly when the mirror is to be utilized in airborne or space-based applications.